Here's what I charge:
4x6: $5
8.5x11: $15
But I try to NOT do big prints at the track. Except for the winternats this year, i took my i9900 to the race and cleaned up on selling framed 8x10's for $40.
I also brought my studio gear and did a studio of the car and put an action shot in the corner like this:
I must have sold like 20 of them that week.
Then I did this for the track owner (the action shot was supposed to be normal until I caught him in a big wreck!)
(edited, i'm a tool and didn't realize you meant how much you should charge)